tests: don't run couple of tests related to extensions loading with chg The couple of files test extensions loading and debugging that. It sets `devel.debug.extensions` to True to do that. Either we have to restart chg if this config is set or just don't run these tests on chg. I tried the first way by adding the config option to chgserver.py list of config subsections but that does not seem to work.

tests: conditionalize test-phases.t output for chg IIUC chg loads the ProgrammingError class which leads to it directly referencing it in error output. This makes the test pass on chg.

chg: be stricter about checking invocation of `serve` command Few tests run serve command in form of `hg -R <repo> serve` which leads to chg thinking that it's not a serve command and failing tests. We become more stricter in checking for the serve command. This fixes test-server-view.t, test-remote-hidden.t, test-remotefilelog-http.t, test-phases-exchange.t, test-wireproto-content-redirects.t with chg.